Characterization of a Block Copolymer with a Wide Distribution of Grain Sizes

Block copolymer/lithium salt mixtures are an emerging class of lithium battery electrolytes. Previous studies have shown that the ionic conductivity of these materials is a sensitive function of grain size. Both depolarized light scattering (DPLS) and small-angle X-ray scattering (SAXS) have proven to be effective techniques for elucidating the grain structure of block copolymer (BCP) materials. DPLS is particularly useful for the characterization of samples with grain sizes larger than 1 μm, whereas SAXS is particularly well suited for samples with grain sizes smaller than 0.1 μm. We present the results of both DPLS and SAXS measurements of grain structure in a BCP/lithium salt mixture that was annealed after being initially prepared by freeze-drying from solution. The combination of the two techniques demonstrates that our sample is characterized by an extremely wide distribution of grain sizes. In particular, the sample has a large population of small sub-micrometer-sized grains that cannot be detected optically. A bimodal grain distribution model is presented to support this interpretation of the experimental data. The presence of both large grains and regions of undetectable small grains was confirmed by polarized optical microscopy (POM). Two-wavelength DPLS measurements provide an additional approach for characterizing block copolymer samples with a broad distribution of grain sizes.